<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btnId">id</button>
    <button class="btnClass">class</button>
    <button a = "1">a1</button>
    <button a = "2">a2</button>




    <script src="./javascript/选择器.js"></script>
</body>
</html>